The present invention relates to contact detection in medical diagnostic equipment. It finds particular application in conjunction with nuclear or gamma camera systems and will be described with particular reference thereto. However, it should be appreciated that the invention will also find application in conjunction with other equipment where contact detection is desirable. Nuclear or gamma cameras include one or more detector heads which each receive radiation emanating from a patient. Each head includes a flat scintillation crystal which converts incident radiation to flashes of light. Internal electronics convert each flash of light into an indication of the location and energy of the received incident radiation event. Collimators are commonly mounted to the face of each scintillation crystal. Collimators are designed with lead vanes to limit the radiation received by the scintillation crystal to only that radiation which is coming essentially perpendicular to the face of the crystal. The height of the vanes and their spacing control the angle at which received radiation may differ from perpendicular. Different collimators are designed for different types of medical procedures. Collimators and associated scintillation crystals are secured to a mounting structure which in turn is secured to a means which provides for horizontal, vertical and rotational movement of the detector head assembly. The remainder of the head assembly is constructed to support the lead vane collimators thus the entire head assembly tends to be massive. In a typical clinical application it is desirable to position the detector head assemblies in close proximity to the portion of the patient to examined. To facilitate this positioning, head assemblies are mounted to a gantry system with the face of each detector disposed adjacent to an area of patient interest under examination. Gantry systems are generally perpendicular to the floor surface and typically provide a means for rotating detectors about an isocenter that is parallel to the floor. In multiple head assembly systems, detectors are typically positioned on the gantry at uniform angular intervals. In application, a patient under examination will lie on an examination table parallel to the floor. The patient is positioned within the gantry such that the patient to be imaged is near the gantry isocenter and each detector face is exposed to the portion of the patient to be imaged.
To optimize the imaging capability of a camera system a means is typically provided which allows the operator to position each head assembly radially toward the gantry isocenter. Additional movement capability is provided to pivot or cant each detector head assembly independent of radial movement. The radial movement means is typically a motor and gear arrangement which is controlled by the camera operator. In multi-head camera systems each head may be operatively connected to other heads in the system such that movement of all the heads occurs simultaneously. Alternatively, heads may also be selectively positioned using independent motor and gear arrangements for each detector head.
Because detector head assemblies are massive it is necessary that the gantry, the head assembly supporting means and the motor and gear means be ruggedly designed and powerful enough to safely position the head assemblies in close proximity to the patient. Because of the gantry size, the power associated with the motor and gear arrangement and the mass of the head assemblies, there is concern over patient or patient couch contact from the head assemblies during head positioning. Contact of this type is generally unintended and may result in patient injury, patient table damage or head assembly damage. To interrupt head movement in the event of unintended contact by the head assemblies, a contact sensor means is operatively positioned on the head assembly face to detect when the assembly has made contact with an object, such as a patient or the patient couch.
Known contact sensor means consist of two electrically conductive sheets in close proximity spaced parallel relationship. The conductive sheets are separated by a plurality of firm electrically non-conductive spacers fixedly positioned between the conductive sheets such that sheets are in opposition but not in electrical contact. The quantity and placement of spacers between the conductive sheets and the spacers individual sizes are selected to (1) maximize the exposed surface area between the conductive sheets, (2) facilitate a close proximity spaced parallel relationship between the opposing surfaces of the conductive sheets and (3) maintain electrical isolation between the conductive sheets while no contact force is applied to the outer surface of the contact sensor. The conductive sheets are constructed from materials which allow displacement of the sheets in a direction normal to the surface of said sheets in response to contact forces applied to the outer surface of the contact sensor in areas between adjacent spacers. In this fashion a means is provided for the conductive surfaces to make electrical contact in the areas between the spacers. After the contact forces are removed the sheets will return to their original non-electrically contacting, close proximity spaced parallel relationship. The conductive sheets are electrically connected to a control means which senses when the sheets have made electrical contact and interrupts the advance of head assembly in response. This spacer and sheet arrangement is secured to a mounting frame. The entire assembly is secured by suitable fastening means across the face of the detector head assembly exposed to the patient. The conductive sheet mounted adjacent the detector face is alternatively described as the inner conductive sheet whereas the conductive sheet intended to initially receive contact forces is alternatively described as the outer conductive sheet. The fastening means, electrically conductive sheets, spacers and mounting frame are constructed from various materials which allow radiation events emanating from the patient to pass through to the collimator and scintillation crystals. In theory, if the known sensor means contacts an object while the detector head assemblies are in motion, the outer conductive sheet will deform towards the inner conductive sheet and make electrical contact. This electrical contact is sensed by the control means which stops the advance of the head assembly. Typically, a graphics layer is laminated to the outer conductive layer for aesthetics.
The contact sensor means described above is adequate in many situations. However, if the contact sensor means contacts a hard protrusion, such as a patient's knee or elbow, at a point on the conductive sheets where a rigid spacer is situated or if the surface which the head assembly contacts is a relatively broad and firm area parallel to the face of the sensor means, such as a male patient's chest, the two conductive sheets will not make contact until there is sufficient force applied in areas between the spacers to deform the sheets thereby initiating contact between the sheets. In some instances no contact is made at all. During the time it takes for sufficient force to be applied to deform the sheets about the spacers and initiate electrical contact between the sheets, the detector heads will continue to advance possibly injuring a patient, damaging the detector head and/or patient couch. To minimize this problem it has been proposed to reduce the thickness of the non-conductive spacers. While this approach increases sensitivity it also results in reduced reliability due to the conductive layers contacting as a result of momentive forces acting upon the conductive sheets during initial movements of detector head assembly without actual contact to an object. Another alternative is to increase the distance between adjacent spacers however this approach suffers from the reliability problem previously described. Neither of these solutions fully overcomes the contact sensitivity problem described above.
The present invention contemplates a new and improved contact sensor means which overcomes the above-referenced problem and others.